1. Field of the Invention
This invention relates to a method for refining high carbon steel by top and bottom blown process, and more particularly to improvements in and relating to the production of high carbon steel with 0.3% or more C-content by the top and bottom blown refining method in which pure refining oxygen is blown against the surfaces of molten iron through a top blowing lance nozzle while an inert gas is blown into the molten iron through a nozzle at the bottom of a furnace for stirring the melt.
2. Description of the Prior Art
In refining iron, the main trend in the current steel-making industry is toward the LD converter using top-blown pure oxygen. In this connection, however, various studies have been developed and devoted to a top and bottom blown refining method incorporating the characteristics of the bottom blown oxygen process. According to this method, the entire amount or major portion of pure oxygen which is to be used for refining molten iron is blown against the surfaces of the molten iron through an oxygen blowing lance in a manner similar to the LD converter, while an inert gas is blown in through a gas blowing nozzle (or a tuyere) which is provided at the bottom of the converter, solely or along with slight amounts of oxygen and cooling gas. The latter is usually called "bottom blown gas" and serves to improve the metallurgical reactions to a considerable degree by its function of stirring and strengthening the molten iron and slag.
Although the flow rate of the bottom blown gas is extremely small as compared with that of the top blown gas, the method has the advantage that the refining speed can be controlled over a wide range by adjusting the oxygen flow rate by the top blown gas or the height of the blowing lance, due to its excellent refining reaction characteristics. The flow rate of the bottom blown gas itself is an important factor which largely influences the precision of the control of the refining process. However, no knowledge has been brought to the attention of those skilled in the art with regard to its influences or possibilities of improvement of the high carbon steel except that a greater flow rate of the bottom blown gas is reflected by a reduced O-content and an increased Mn-content in the steel and smaller (T.Fe) contents in the slag as obtained at turn down. Further, there have been provided no feasible data with respect to the kind of bottom blown gas and the bottom blowing of part oxygen gas which may cause a substantial change in the control of the refining process.